In the fields of using natural gas and steam as raw materials to produce hydrogen gas, methanol, syngas and synthetic ammonia, and in the various technologies, stringent control on the proportions of these two raw gases, i.e. the steam to carbon ratio (mainly the total number of carbon atoms and volume flow of steam), is required. As the main composition of natural gas is methane, the steam to carbon ratio in the above technologies refers to: the ratio of volume of methane to volume of steam under the same temperature, pressure and operation conditions.
Steam to carbon ratio is generally calculated from the average carbon molecular numbers of raw materials in a device. When the raw materials change greatly, the lack of calibration leads to inaccuracy. The steam to carbon ratio is the most sensitive technical parameter in light oil reforming process. A rise in steam to carbon ratio can reduce coking of catalyst and effective surface area of conversion catalyst and decrease the reaction activity and efficiency, and decrease C2 or higher hydrocarbon content at the bed outlet, all of which are very beneficial to the reformation reaction. However, the rise in steam to carbon ratio correspondingly increases consumption of desalinated water and reformer energy, and increases load of the downstream cooling system. Therefore, a suitable steam to carbon ratio can only be determined according to a specific industrial device.
However, in the present technologies of applying natural gas for hydrogen, methanol, syngas and synthetic ammonia production, only a flow rate control system is provided on pipelines of the two raw gases for controlling steam to carbon ratio. Realization of such steam to carbon ratio control technology completely relies on the linkage between adjusting valves and flowmeters. Since response time of the adjusting valves towards signals produces different levels of delay or lagging and flowmeters for measuring flow rate inherently have measurement errors, accurate flow rate control is unable to be realized and the flow rate always fluctuates within a certain range. Such would unavoidably result in the inability of the adjusting valve to timely adjust the flow rate, and in turn fail to achieve accurate and stable control on steam to carbon ratio. In addition, different flowmeters have high demands towards installation positions.